Attempts to end the chaos inside Europe's emissions trading scheme (ETS) stumbled today when the market reopened, only for minimal trading to take place.

Traders were said to be worried that business could remain polluted by the theft of carbon credits in Austria and elsewhere that forced a shutdown of the scheme on 19 January, at the estimated cost of £90m in lost business.

The European commission has called on national carbon registries to beef up their IT security systems, but has upset traders by declining to publicly reveal the minimum standards now required.

The ETS is seen as a vital tool in the fight against climate change and the fraud is a setback to attempts to sell the cap-and-trade scheme to the US, Australia and elsewhere.

Britain, France and three other countries resumed trading but it took three hours before any trades had been reported on the Paris-based BlueNext platform, while larger exchanges such as that of London-based ICE remained shut.

New York's Green Exchange, backed by JP Morgan and Goldman Sachs, also remained closed for business, with officials there saying it could weeks before they felt confident the system had been suitably cleansed.

"In our view there are still too many unanswered questions. In particular we are concerned that the Austrian government has not formally confirmed a complete list of [stolen] serial numbers," said Henrik Hasselknippe, a managing director at Green Exchange.

The International Emissions Trading Association (IETA) said it was pleased that some spot trading had resumed but remained wary about whether the wider problem had been solved.

"Given that the minimum security requirements recently agreed by European authorities to lift a registry suspension remain confidential, only time will tell if they are sufficient." said Henry Derwent, IETA's president today.

"At this stage, it seems that some registries will reopen with components of the IETA-recommended double approach – requiring both reinforced security of electronic transactions and two-factor authorisation to finalise a transaction – missing."

The association acknowledged the importance of government efforts to reopen registries and co-ordination with the European commission, but it remained concerned about the "continuing lack of clarity on the legal and criminal implications" of the allowance thefts.

Trevor Sikorski, carbon analyst with Barclays Capital, said there was a need for urgent new rules limiting market access to carbon producers and regulated financial intermediaries, adding: "You'd have to be a brave man to trade that stuff."

While short-term "spot" trades have been hit by the fraud and closures, carbon futures trading has continued without interruption. Analysts at Point Carbon Thomson Reuters estimate the value of lost spot trading since 19 January to be in the order of €110m.